The so-called "toadstone" of Derbyshire had long been known to the local miners, but it first came to public attention when John Whitehurst wrote about it, and mapped it, in 1778. Whitehurst had written a speculative book about the origin of the earth, but he appended to his treatise a much more matter-of-fact discussion of the stratigraphy of Derbyshire. He was intrigued by toadstone, which was found in three separate layers, between strata of limestone. Most people assumed that toadstone was sedimentary, since it was found between other sedimentary formations. But Whitehurst noticed that the toadstone layers were of a curious kind of rock resembling basalt, often contained air-pockets, and tended to vary in thickness. He concluded therefore that toadstone was the product of a volcano, which had poured its lava between the layers of already existing limestone. It was an astonishing intellectual leap, considering that Whitehurst had never heard of Desmarest and had no other reason to suspect that basalt might be volcanic.

The plate shows a section near Matlock High Tor, north of Derby. The layers numbered "4", "6", and "8" are the three toadstone formations. The location of Matlock High Tor can be seen on the maps of John Farey (exhibit item 45) and William Smith (exhibit item 46).